A known structure for a conventional lens assembly comprises a receiving part formed on a lens frame, for positioning the lens along the optical axis, and a plurality of locking parts having elasticity and extending along the optical axis, formed at a plurality of places (e.g. three sites) of a wall surrounding the outer periphery of the lens supported by the receiving part, wherein the lens is held to be supported elastically between the locking part and the receiving part (see, for example, JP 2007-65017A). Similar structure is also known, in which a plurality of lenses stacked each other and retained elastically interposed between a receiving part and a plurality of locking parts formed on the lens frame (see, for example, JP 2007-121612A).
A fish-eye lens (wide-angle lens), having an imaging angle over 120 degrees, is commonly used for an on-board camera so as to acquire images of a wide range. This type of lens is commonly applied to a lens assembly having a plurality of lenses, adapted to a lens frame having apertures open to both its subject side and image side and disposed in serial along the optical axis (see, for example, JP7-10710U).
As such lens assembly, for example, a known structure comprises a receiving part formed at one end of a lens frame, for positioning the lens along the optical axis, wherein a plurality of lenses are inserted onto the receiving part to be stacked sequentially from the smallest to the largest according to respective diameters, such that the surface edge of the topmost lens is locked by a locking part formed on the other edge of the lens frame (see, for example, JP 2007-121612A).
However, both conventional lens assemblies described in JP 2007-65017A and JP 2007-121612A provide flanges on the lens which the locking parts of the lens frame is to abut with, so that the locking parts contact these flanges. To this end, the lens diameter including the flanges increases and the outer diameter of the lens frame is increased accordingly, thereby possibly increasing the whole size of the lens assembly, as well. Moreover, since flanges are formed on the lens, complicated lens processing is necessitated and thus the cost will grow.
Three locking parts are formed on the lens frame, for example, whereby it is difficult to gain sufficient support strength. For this reason, the lens may be displaced within the lens frame by an external impact, and as a result, for example, the imaging optical axis may shift in a case of an imaging lens, while the projection optical axis may shift in case a projection lens, whereby desired images may not be acquired in either case.
The conventional lens assembly as in JP 7-10710 U may not be assembled efficiently and accurately because the lens assembly has a large number of parts and the structure thereof is inevitably complicated.
Moreover, in the lens assembly disclosed in JP 2007-121612A, mentioned above, the circumferential surface of each lens is formed in parallel with the optical axis (in the thickness direction). That is, the circumferential surface of each lens is formed to have equi-diameter in the thickness direction. The inner circumference of the lens frame is also formed stepwise in the optical axis, so as to correspond with the diameter and the circumferential shape of each lens.
As a result, when inserting each lens to the lens frame, the lens may fall over or inserted with its optical axis being tilt with respect to the predefined optical axis set for the lens frame because the outer peripheral surface of the lens may be caught by the inner peripheral surface of the lens frame having a corresponding inner diameter thereto, and therefore, it is difficult to insert the lens in the predefined posture. Especially, when inserting plurality of lenses to the lens frame such that these lenses stack each other, if the lens inserted prior to the other lens is tilted to the predefined optical axis, the next lens will also be inserted with a tilt, and thus the optical performance will be deteriorated.
The lens frame has an inner circumferential surface stepwisely formed along the optical axis in accordance with the diameter and shape of circumferential surface of each lens. Therefore, in a case where a predetermined thickness corresponding to such a stepwise inner circumferential shape is ensured for the lens frame, the portions having large external diameters increase, whereby the whole dimension of the lens assembly may become larger.